1. Field of the Invention
This invention relates to a pressure gauge, more particularly to a tire pressure gauge having correction means to correct the position of a pressure responsive plunger.
2. Brief Description of the Related Art
Conventional tire pressure gauges are generally provided with a pressure responsive movable indicator rod to indicate the pressure value that is detected thereby. Referring to FIG. 1, a conventional tire pressure gauge comprises a tubular housing 10, a pressure measuring head 11, a piston 12, a spring 13, a hollow guide 14, and a plunger or graduated pressure indicator 15. When measuring the pressure inside a tire, the pressure measuring head 11 is connected to an inflating valve (not shown) of the tire so that the gas in the tire enters the pressure gauge and pushes the piston 12, which in turn moves the pressure indicator 15 out of the tubular housing 10 against the biasing action of the spring 13. The pressure inside the tire is read from the graduation of the pressure indicator 15 which is aligned with a pointer, i.e. the bottom end of the hollow guide member 14 or the bottom end of the tubular housing 10. After reading, the pressure indicator 15 is pushed inward manually to revert to its original position.
In manufacturing such piston-type pressure gauges, springs with a proper design are chosen, and graduations are provided on pressure indicators based on a predetermined relationship between the force of the spring used and the amount of the spring displacement. It is necessary that the springs are produced with uniform quality so that a precise pressure value can be read from the pressure indicators. However, in practice, quality deviation often occurs in producing the springs because it is difficult to obtain uniform quality of springs even under the same controlled condition. Since unqualified products are discarded after a stringent quality inspection, the cost of manufacturing such pressure gauges usually cannot be kept low. An approach to reduce the manufacturing cost is to reduce the amount of springs to be discarded by allowing a wider tolerance range for the springs and by providing correction means for maintaining the precision of the pressure indicators.